The present invention relates to improvements in the ignition of sparks between electrodes defining a gap and more particularly the invention relates to the improvement in the operation of underwater spark gaps used for the production of shockwaves serving for the contactfree comminution of concrements in the body of living beings.
Shockwave sources are used in a variety of medical and technical equipment. Here particularly shockwaves have been found highly suitable in shockwave lithotripsy for the noninvasive destruction of concrements in the body of living beings. Basically electrical energy stored in a capacitor is discharged in an underwater spark gap and on the production of the discharge spark or arc the local sudden heating produces shockwaves. The shockwaves are then focused towards a concrement, pass through the skin of the patient and combine in the focal point of the equipment that has been oriented to coincide with a concrement. The concrements are reduced in this fashion to small gravel and fractions and can then be discharged through normal physiological process.
The shockwave focusing is usually carried out under utilization of a reflecting rotational ellipsoid having two focal points; one of them contains (or straddles) the spark gap and the other one is positioned to coincide with the concrement in the person. U.S. Pat. No. 3,942,531 as well as the German patent 26 35 635 shows various forms of the spark gap. For further reference see also U.S. Pat. Nos. 4,809,682, 4,940,050, 4,905,673, 4,938,781.
Considering some details of a discharge into a gap, the path of the arc in the gap is determined through a near currentless path of a so called leader. This leader is particularly a channel between the two electrodes and is produced in the instant of applying a high voltage between the electrodes but prior to the actual current flow and that leader then determines the current flow that forms the spark and is the actual arc. The leader is primarily determined by the field gradients and field lines between the positive and negative electrodes. But local variations on account of the presence for example of water or the like determines considerably the local detailed path configuration of that leader. In other words, a straight line between say the electrode tips is more or less an average path approximation. The electrical field needed between the electrodes for producing an adequate shockwave that is sufficient for the destruction of concrements could lead to thermal breakthrough characterized by certain delays in the ignition lasting from 1 microsecond up to a millisecond as between the ignition triggering and the actual spark depending on the voltage, the effective conductivity in the distance and other geometric factors. The relatively large temporal spread is attributed to the fact that the growth and propagation of the leader is a stochastic process, but that spread in the delay results in significant variations in the level of shock wave production.